Today, energy efficient windows are being used to help reduce the energy requirements for both heating and cooling of indoor spaces. Typical of such energy efficient windows are double pane windows separated by a spacer, the space filled with a thermally insulating gas such as air, argon, or xenon. These windows can be coated with static films for blocking near infrared (NIR) light, thus reducing the heat passing through the window. Other windows, tailored for use in colder climates, can be coated with films that block out far infrared light to help retain heat. One disadvantage of such coatings, however, is that they cannot be dynamically changed to accommodate changing weather conditions.
Electrochromic (EC) materials have been suggested for use with energy efficient windows, these coatings able to reversibly switch their optical properties when a voltage is applied. Presently, tungsten oxide (WO3) is the most studied electrochromic coating for commercially available windows. Nevertheless, when WO3 switches, it modulates mainly the visible part of the light spectrum (540-750 nm), while the near infrared (NIR) light spectrum (750-2500 nm) remains either unchanged or switches simultaneously with the visible.
As another drawback, previous TCO nanocrystal-based electrochromic devices have been implemented using a liquid electrolyte. Typical liquid electrolytes are volatile and can leak. As a result, a liquid electrolyte is not suitable for a large scale commercial device.